882 
Journal of Agricultural Research voi. xxiii. No.« 
Table III .—Effect of length of the daily light period on the size of seed in different varieties 
of soybeans 
Average weight of 500 seeds. 
Length of daily light exposure. 
Man¬ 
darin. 
j Peking. 
Tokyo. ; 
Biloxi. 
Germinated May 17: 
Gift. 
j 
j Gm. 
Gm. i 
Gm 
Full daylight throughout, 15 to 
11J/2 hours. 
IOO. O 
1 43 * 2 
IIO. 4 | 
Did not mature. 
7 hours throughout. 
67. 6 
48.4 
130.6 j 
13 1 * 2 
5 hours till 1 week after flowering; 
j 
full daylight thereafter. 
85.8 
36. 0 
60. 4 ‘ 
88. 4 
Full daylight till flowering; 7 
1 
hours thereafter. 
i . ! 
! 49 - 2 
Full daylight till flowering; 5 
1 i 
1 1 
i 
hours thereafter. 
37 *o 
rot;. O'.. 
Germinated June 15: 
Full daylight throughout, 15 to 11KI 
1 
* 
hours.j 
93.8 | 
37*4 
99.8 j 
Did not mature. 
12 hours throughout. 
89.4 1 
62. 8 
125.8 | 
129.8 
Darkness 10 a. m. to 2 p. m. (and 
at night) throughout. < 
3°. 0 
J 
j 
It has been found, also, that the size of the individual seed is influ¬ 
enced by the relation of the prevailing light period to the respective 
optimal period for vegetative and reproductive development. The 
data presented in Table III throw light on this question. It is seen that 
in the early variety of soybeans, Mandarin, which fruits readily in the 
longest days of summer, shortening the light period to 12 hours or less 
reduced the size of the seed and restoring the plant to the full summer 
daylight period after flowering had occurred tended to offset the unfavor¬ 
able influence of the previous exposure to 5 hours of light daily. With 
the later varieties, requiring shorter days for flowering, the results are 
quite different. For these a 12-hour exposure gives decidedly larger 
seeds than the full day length of summer and for the Tokyo and Biloxi a 
7-hour day gives fully as large seed as a 12-hour day. A 5-hour light period 
till flowering, followed by exposure to the full day length, reduced the 
size of the seed markedly in all except the early variety. A midday 
period of darkness acted unfavorably. All these data harmonize with 
previous results on the action of regulated day lengths in initiating or 
inhibiting flowering and fruiting in these varieties of soybeans. 
An intermediate light exposure tends to delay the time of flowering, and 
as the light period is advanced toward the optimum for vegetative devel¬ 
opment flowering is more and more delayed, the tendency being toward 
more or less indefinite inhibition. Biloxi soybeans which germinated 
June 1 began flowering June 27 under a io-hour day, while the first 
blossoms appeared July 13, 16 days later, under a 13-hour day; Other 
plantings gave very similar results. It has previously been shown that 
a day length of 14 hours or more delays flowering indefinitely in the 
Biloxi. The Peking variety planted on June 18 began flowering July 14 
under a 13-hour day as well as under a io-hour day for the reason that 
the 13-hour day length does not materially exceed the optimum for 
flowering in this variety. It should be noted, however, that develop¬ 
ment and ripening of the seed of the Peking are decidedly hastened by 
